This proposal requests support for the establishment of a Human Twin Panel at the Medical College of Virginia that will be a resource at this institution for a wide variety of clinical studies. Three types of research projects will be conducted: Studies of quantitative inheritance in man, studies utilizing the co-twin control method and studies dealing with the biology of twinning. The genetic control of blood pressure will be investigated by a classic twin study of renin and aldosterone metabolism. Biochemical, anthropometric, genetic and psychological variables will be studied using a new genetic model based on the analysis of the half-sib offspring of identical twins. Genetic and environmental predispositions to cancer will be studied in idential twins with neoplastic disease. The effects of pharmacologic doses of Vitamin C will be measured in a double blind co-twin control study. Evidence will be sought for the existence of polar body twins and superfetation in man in a study of newborn twins that will also document the significance of placental variation as a cause for intrapair differences in later life. Finally, the cause of dizygotic twinning will be investigated by endocrine studies of the mothers of dizygotic twins.